The invention relates to the field of simultaneous two-way transmission of information on a same communication channel, that is duplex data transmission. It more particularly relates to an echo canceller for such a system, of the type having an adaptive digital filter.
Before the state of the prior art is considered and the invention is summarized, it may be useful to give some indications regarding duplex transmission of data on a common communication channel and to outline the problems which are involved.
Referring to FIG. 1, a system for simultaneous two-way communication between two remote stations A and B on a single communication channel 10, for instance a two-wire telephone circuit, is schematized. The data to be transmitted consist of a sequence of symbols which are typically quantified, which may represent data signals as well as speech signals. When the useful signals a and b from stations A and B are transmitted in the same frequency range, the signal r which is received by receiver 12 at station A includes the useful signal b sent by transmitter 11 of the remote station B, and an additive noise x: EQU r=b+x (1)
The noise x may frequently have power much greater than that of the useful signal b. It comprises an echo of signal a transmitted by source 11 at station A, although differential transformers 15 are provided at both ends of the transmission channel 10. That phenomenom is indicated in FIG. 1, where transmission from B toward A is indicated in full line, while transmission from A toward B is indicated in dash-dot lines.
Since the echo signal b may prevent correct recovery of signal b by receiver 12 at station A, its action should be cancelled. There exists a number of approaches for cancellation of the echo. Reference may for instance be had to the following documents for finding examples: Mueller, "A New Digital Echo Canceller for Two-wire Full Duplex Data Transmission", IEEE Transaction on Communications 24, No. 9, 1976, pp. 956-962; French Pat. No. 2,377,734; Weinstein, "A Passband Data Driven Echo Canceller For Full Duplex Transmission on Two-wire Circuits", IEEE Transaction on Communications, 25, No. 7, 1977, pp. 654-666; Falconer et al "Adaptive Echo Cancellation/AGC Structures for Two-wire Full Duplex Data Transmission", The Bell System Journal, Vol. 58 (1979), pp. 1593-1616; U.S. Pat. No. 3,780,233 (Campanella et al).
As a general rule, it has been proposed to locate an echo canceller (which will be referred to in the following by the abreviation ECC) in the receive line. That ECC is an adaptive digital filter 13 whose transfer function may be represented with a vector H which, from a sequence A of successive symbols a.sub.k (where k indicates the serial number of the symbol) transmitted by source 11 of station A, delivers a linear estimation: EQU y=H.multidot.A (2)
That estimation y may be considered as a reconstruction of the actual echo x. The reconstructed echo is applied to a subtractor 14 which also receives the signal r arriving to station A from the far end on line 10. The difference between the two signals is applied to the receiver 12.
The echo x may generally be considered as comprising a close echo x.sub.p due to the lack of adaptation of the differential transformer 15 of station A and a remote echo x.sub.L due to reflections of the signal a transmitted from A toward B, reflections which are due to lack of impedance adaptation in the communication channel 10: EQU x=x.sub.p +x.sub.L ( 3)
The two echo components have different features. The close echo is much more powerful than the remote echo. And the features of the two echo components and the parameters of the useful signal exhibit large variations depending upon the communication channel which is considered.
It is important to note that the power of echo x is quite variable and its value is unknown. Under actual conditions, the only available indication is the fact that the value is lower than a predetermined maximum level. The power of the useful signal b is also quite variable and the only available indication is the fact that it is higher than a predetermined minimum level. Last, the remote echo x.sub.L is fequently affected with a phase shift, due to phase jitter and a frequency shift in the communication channel, while the close echo is typically not subject to phase shift.
When the ECC consists of an adaptive digital filter of conventional type, it must comprise a large number n of bits on each tap coefficient for the correction to be satisfactory, even if there is no phase shift of the remote echo. Most prior art ECCs typically have twenty bits per tap coefficient, with the consequence that they are complex and of high cost. The need for a large number of bits is particularly due to the broad dynamic range of the echo and the useful signal.
It is an object of this invention to provide an echo canceller for data communication systems which makes it possible to substantially decrease the number n of bits of each coefficient, without detrimentally affecting the performances, particularly the acceptable dynamic range of the echo and useful signal.
When the remote echo exhibits a phase shift, there is another problem which is not overcome in the prior art. As will be more apparent in the following, the ECCs of the prior art cannot operate properly when the echo has a phase shift, unless the power of the echo which is not in phase is known. The performance is detrimentally affected when the power of the echo which exhibits a phase shift increases.
It is another object of the invention to provide an echo canceller which is more effective than the prior art ECCs when an echo exhibiting a phase shift is present.
According to an aspect of the invention, there is provided an echo canceller for a full duplex communication system between two remote stations, comprising an adaptive digital filter and automatic gain control means located downstream of said filter in the communication path, and means for delivering the value of the gain of said automatic gain control means to said filter as an input signal thereof.
In other words, the adaptive digital filter is "looped" on the AGC means, since the gain of the AGC means is fed back to the adaptive filter as an input signal which cooperates in determining the vector of the coefficients of the filter.
Means controlled by the output signal of the adaptive filter will be provided for adjusting the coefficients of the latter for maintaining the power or amplitude level of said output signal in a predetermined range. Such means simultaneously modify the coefficients of the filter and the gain of the AGC means, in opposite directions.
The invention will be better understood from the following description of particular embodiments of the invention and a comparison with the prior art.